Commercial switchable glazing devices, also commonly known as smart windows and electrochromic window devices, are well known for use as mirrors in motor vehicles, aircraft window assemblies, sunroofs, skylights, and architectural windows. Such devices may comprise, for example, active inorganic electrochromic layers, organic electrochromic layers, inorganic ion-conducting layers, organic ion-conducting layers and hybrids of these sandwiched between two conducting layers. When a voltage is applied across these conducting layers the optical properties of a layer or layers in between change. Such optical property changes typically include a modulation of the transmissivity of the visible or the solar sub-portion of the electromagnetic spectrum.
The broad adoption of electrochromic window devices in the construction and automotive industries will require a ready supply of low cost, aesthetically appealing, durable products in large area formats. Electrochromic window devices based on metal oxides represent the most promising technology for these needs. Typically, such devices comprise two electrochromic materials (a cathode and an anode) or sometimes a coloring and a charge storage layer, separated by an ion-conducting film and sandwiched between two transparent conducting oxide (TCO) layers. In operation, a voltage is applied across the device that causes current to flow in the external circuit, oxidation and reduction of the electrode materials and, to maintain charge balance, mobile cations to enter or leave the electrodes. This facile electrochemical process causes the window to reversibly change from a more bleached (e.g., a relatively greater optical transmissivity) to a more darkened state (e.g., a relatively lesser optical transmissivity).
For long-term operation of an electrochromic window, the components within the device must be well-matched; e.g., the electrochemical potentials of the electrodes over their states of charge should be within the voltage stability window of the ion conductor and of the TCO material. If not, electron transfer will occur between the electrode materials and the other window components causing, for example, leakage current, electrolyte consumption, buildup of reaction products on the electrode(s) and, in general, significantly shortening the useful lifespan of the window.
TCO materials typically used in electrochromic windows such as FTO and ITO react with lithium at voltages below ˜1V vs. Li/Li+, lowering their electrical performance and darkening the material. Electrolytes typically incorporated into the ion conductor, or the presence of water or protic impurities, have voltage stability windows between ˜1 and ˜4.5 V vs. Li/Li+. Therefore, it is beneficial to use electrode materials that undergo redox events within these limits. For example, lithium nickel oxide (LiNiOx) is an anodic electrochromic material that is bleached at about 2.5 V vs. Li/Li+ and darkens upon oxidation, typically to about 4.0 V vs. Li/Li+.
Certain tungsten oxide based materials darken cathodically to produce a darkened state transmission spectrum that is complementary to LiNiOx and therefore can be partnered with LiNiOx in electrochromic windows. Certain methods for the preparation of WO3 thin films have been reported in the literature.
Tungsten oxides are well-known EC-active materials. Uncertainty exists in the literature, however, regarding whether crystalline or amorphous materials are preferred. In addition, while tungsten trioxide (WO3) crystallizes in several polymorphs, there is no clear preference as to which polymorph is best, or whether demonstrable differences should be expected. Crystallinity, the degree of crystallinity, and the crystal system obtained varies with synthesis method, temperature, the use of additives and other considerations. One of the crystal phases of WO3 that has been studied by certain processing techniques is hexagonal WO3 (i.e., h-WO3). There are some examples of hexagonal WO3 (i.e. h-WO3) for battery (i.e. electrochemical) and EC applications in the prior art. Commonly utilized synthetic methods such as PVD and electrodeposition, however, are not always amenable to the preparation of single phase, crystalline h-WO3. Instead, h-WO3 has been produced via hydrothermal synthesis, growing nanostructures directly on substrates, and producing nanostructures in solution.
Other crystal phases of WO3 have other symmetries and may be described as triclinic WO3 or monoclinic WO3 or cubic WO3, as is appropriate based on their symmetries. In general, if the composition is the same but the arrangement of the atoms in the crystal phase is similar but differ in symmetry, the crystal phase may be described strictly by its symmetry. In certain circumstances, however, the arrangement of the atoms in the crystal phase may be unique beyond simple distortions that alter the lattice symmetry. In such cases, the use of structure types in addition to a symmetry descriptor is useful.
Tungsten oxide thus may be described as displaying a number of polymorphs. The term polymorph describes symmetry changes that largely maintain some or all of the same atomic connectivity and unique relationships of atoms that produce unique structural features. Sometimes, however, polymorph may describe an entirely different structure and atomic arrangement but with the same composition. Critically, the synthesis and/or thin film deposition method can impact the resulting polymorph. For instance, thermally evaporated tungsten oxide is commonly amorphous especially if the substrate is not heated. The resulting films can be crystallized by post-deposition annealing (e.g., in air), however, the resulting crystal structure of the tungsten trioxide so produced is typically monoclinic perovskite. Monoclinic perovskite tungsten oxide is known to undergo phase transformations upon intercalation (e.g. with Li). [Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; Chapter 23, pp 246-253, American Chemical Society: Washington, D.C., 1963.] Examples of tungsten trioxide materials that typically occur in different structures or symmetries are sol-gel prepared materials and commercially available nanostructured materials which typically have the “Perovskite Tungsten Bronze” [or PTB] structure. The PTB structure may be described as similar to ReO3 in which metal (M) ions (usually monovalent) are intercalated into interstitial spaces of the ReO3 structure resulting in MxReO3 and the perovskite structure type. Also, it is well known in the literature that thermally evaporated films that have subsequently been crystallized by annealing show worse durability than the amorphous tungsten trioxide thermally evaporated films in electrochromic devices.
Another polymorph of WO3 is the cubic pyrochlore. Sometimes the stoichiometry is represented with waters of hydration and sometimes with hydroxides. Sometimes the stoichiometry is represented with counter ions and sometimes the stoichiometry is doubled, e.g. [—]W2O6. For simplicity, the pyrochlore phase will be described here as part of the WO3 series and explained as a substituted WO3 when additional metals are present.
The term “tungsten trioxide” as used herein refers to a material with the formula AyW1-xMxO3±z.kH2O) and has any crystal structure where A is situated within interstitial spaces and where M is substituted within the W—O lattice. As such, A is often a monovalent species such as a proton, an ammonium ion, and/or an alkali metal and may sometimes be an alkaline earth metal. M is a transition metal, other metal, lanthanide, actinide, electrochromic metal or non-electrochromic metal in octahedral coordination. Under these conditions, x is from 0 to 1, y is from 0 to 0.5, and where z can be 0. A and/or M also comprise more than one element and be expressed as A′a+A″b+A′″c and/or M′d+M″e+M′″f where A′, A″ and A′″ and/or M′, M″ and M′″ are different elements, where a+b+c=y and d+e+f=x. “Tungsten trioxide” can therefore refer to materials comprising atoms other than tungsten and oxygen, including but not limited to, substituted tungsten oxide, substituted triclinic tungsten oxide, substituted monoclinic tungsten oxide, substituted orthorhombic tungsten oxide, substituted tetragonal tungsten oxide, substituted hexagonal tungsten oxide, or substituted cubic tungsten oxide. Furthermore, “tungsten trioxide” can refer to structures comprising hexagonal tungsten bronze, hexagonal tungsten bronze-like materials, tetragonal tungsten bronze, tetragonal tungsten bronze-like materials, pyrochlore materials, pyrochlore-like materials, defected pyrochlore materials, defected pyrochlore-like materials, substituted pyrochlore materials or substituted pyrochlore-like materials.
Although a range of electrochromic cathodic materials have been proposed to date, there is a need for cathode films that can be prepared by simple single-step deposition processes to produce EC cathodes with improved thermal stability, electrochemical/electrochromic durability, high optical clarity in their as-deposited states, and that can be tuned via composition and film thickness to adopt a wide variety of area charge capacities and optical switching properties.
Corresponding reference characters indicate corresponding parts throughout the drawings. Additionally, relative thicknesses of the layers in the different figures do not represent the true relationship in dimensions. For example, the substrates are typically much thicker than the other layers. Unless dimensions are explicitly noted, the figures are drawn only to illustrate connection principles, not to give any dimensional information.